1. Field of the Invention
The present invention relates to a motor pump unit in which a housing has first and second chambers defining an oil reservoir separated by a separation wall, and an electric motor for driving a radial piston pump arrangement within the first chamber.
2. Description of the Related Art
In the motor pump unit disclosed in DE 299 06 881 U, oil returning from a hydraulic system either directly flows into a first chamber or finally flows into the first chamber from a second chamber via a lower open passage in a separation wall between the first and second chambers. In a horizontal operation position of the motor pump unit the same filling level is achieved in both chambers. A ventilation bore situated in an upper portion of the separation wall allows air to pass through. The ventilation bore has a significantly smaller cross-section than the passage in the separation wall. A radial piston pump provided within the first chamber has to generate extremely high hydraulic pressures with small supply rate, e.g., between 700 bars and 800 bars. After long resting periods of the motor pump unit, or when tilting or moving the motor pump unit, air can be trapped in the oil. The intrusion of trapped air cannot be avoided reliably even by using downwardly extending suction tubes for the radial piston pump elements. As the pistons of the radial piston pump elements are made with small diameters, any trapped air leads to a significant decrease of the supply efficiency of the radial piston pump such that a desired maximum pressure cannot be reached.
Further publications relating to motor pump units are GB 20 00 221 A, DE 295 19 941A, DE 39316 99A, EP 0890 741A, and DE 38 39 689 A.
It is an object of the invention to provide such a motor pump unit with increased supply efficiency to reliably reach the needed maximum pressure under all operating conditions.
By a filling and pressure pre-biasing system associated with the first chamber for all operating conditions, not only is the predetermined filling level reliably maintained in the first chamber, but also, a hydraulic biasing pressure is generated at the suction side of the radial piston pump arrangement. The measures significantly increase the supply efficiency of the radial piston pump arrangement and avoids air getting trapped in the radial piston pump elements. The hydraulic pre-biasing pressure also allows the small diameter pump elements to automatically remove occasionally trapped air. Due to the increased supply efficiency extremely high pressures of, e.g., 700 bars to 800 bars can be reached reliably by radial piston pump elements having small pistons and operating with small supply rates. The unit predominantly is developed for operation with horizontal motor shaft (lying working position). However, the concept of the pressure pre-biasing is of advantage also for units operating in upright position.
A spring loaded pre-biasing valve allows an oil exchange from the first chamber into the second chamber via the exchange flow channel only when the adjusted pre-biasing pressure is reached within the first chamber. Air present within the first chamber is transferred via the ventilation channel into the second chamber. The filling level within the first chamber is raised at least up to the height position of the ventilation channel. Oil returning from a return system into the first chamber is under a certain return pressure from which the pre-biasing valve derives the intended prebiasing pressure for the first chamber. The return oil volume furthermore presses residual air from the first chamber via the ventilation channel into the second chamber. Even after a longer resting period of the motor pump unit and/or in case of movements of the unit during transport and/or in case of preliminary strong oil supply demand no air is allowed to entered the radial piston pump arrangement. In case that nevertheless air should be trapped for other reasons the radial piston pump elements even are able to automatically remove trapped air more easily thanks to the pre-biasing pressure within the first chamber. Alternatively or additively the needed pre-biasing pressure and the predetermined filling level also can be achieved by means of a charging pump.
A pre-biasing valve having the form of a screw-in check valve within the separation wall is easy to manufacture and to mount. Screw-in check valves are available for fair costs, only need little mounting space, and are very reliable in function.
Compact dimensions, a stable heat threshold even for permanent operation, and manufacturing the motor pump units for fair costs are possible if the stator winding section of the electric motor designed as an oil immersed motor directly is shrunk into the light metal profile section forming a part of the housing or the oil reservoir, respectively.
At least a flattened section in the periphery of the stator winding part of the motor forms an oil exchange passage through which oil, for example, can be brought to a motor shaft bearing situated remote from the radial piston pump arrangement. Except in the region of the at least one peripheral flattened part, a direct metallic contact is achieved between the stator winding part and a light metal profile section. By the direct metallic contact, heat from the stator winding part is conveyed to the outer side without an insulating oil film between the stator winding part and the light metal profile section. The heat conveyed outwardly then is radiated off by a rib structure and/or is removed with the help of a fan.
The ventilation channel ought to be provided higher up than the suction areas, particularly the suction area located high up of the several radial piston pump elements distributed around the pump shaft. The ventilation channel ought to be located very close to the upper boundary of the first chamber. The oil exchange channel, to the contrary, can be located at the height position of the motor or pump shaft.
A significant improvement of the supply sufficiency already can be achieved by a relatively moderate pre-biasing pressure of about 0.1 bar within the first chamber. Preferably, the pre-biasing pressure is generated by means of the returning oil having the return system pressure. However, alternatively or additively the prebiasing pressure can be generated by means of a charging pump.
A charging pump is preferably driven from the same motor shaft as the other pump sections and is received within the first or second chamber.
Radial piston pump elements having piston diameters between 4 mm and 9 mm are employed to achieve the needed maximum pressure. In this case maximum pressures from about 700 bars to 800 bars can be reached by relatively low driving power. A moderate driving power is preferable for the start-up current of the electric motor in order to allow connection of the motor pump unit as a portable unit to the normal electric power supply without overloading the usual relatively weak fuses usually provided. The motor pump unit is preferably a portable unit with a weight less than about 25 kilos.
In order to simplify the first filling of the first chamber and in order to allow to easily remove the oil a further passage containing a check valve is provided in the separation wall. The check valve blocks in flow direction from the first chamber to the second chamber and opens with relatively low resistance in the opposite flow direction. An oil outlet to a removal screw can be connected to the removal passage, such that the first chamber can be filled when filling the second chamber.
Structurally simple the seat of the check valve directly is formed within the removal passage. A closure ball is co-operating with the seat. The closure ball is secured by means of a securing ring against being lost. Preferably, an oil removal screw can be provided in the second chamber.